By Bob Lewis, AA4PB
An Automatic SealedLead-Acid Battery Charger
This nifty charger is just what you need to keep your SLA
batteries up to snuff!
PHOTOS BY JOE BOTTIGLIERI, AA1GW
A
fter experiencing premature failure of the battery in my Elecraft
K2 transceiver (most likely because I forgot to keep the battery on a
regular charge schedule), I began searching for an automatic battery charger.1, 2
The K2 uses a Power-Sonic PS-1229A
12-V, 2.9-Ah sealed lead-acid (SLA) battery. SLAs are commonly called gel-cells
because of their gelled electrolyte. As
with all things, to obtain maximum service life from an SLA battery, it needs to
be treated with a certain degree of care.
SLA batteries must be recharged on a
regular basis; they should not be undercharged or overcharged. If an SLA battery is left unused, it will gradually
self-discharge.
Although my SLA battery experiences
related here are linked to my K2 transceiver, you can think of the K2 simply as
a load for the battery. The comments pertaining to the SLA batteries and chargers
apply across the board and the charger
described here can be used with any similar battery.
Using a Three-Mode Charger
My first attempt at keeping my K2’s
SLA battery healthy was to purchase an
automatic three-mode charger. I soon discovered that most three-mode chargers
work by sensing current and were never
intended to charge a battery under load.
Three-mode chargers begin the battery
charging process by applying a voltage
to the battery through a 500-mA current
limiter. This stage is known as bulk-mode
charging. As the battery charges, its voltage begins to climb. When the battery
voltage reaches 14.6 V, the charger maintains the voltage at that level and moni1
Notes appear on page 47.
tors the battery charging current. This is
known as the absorption mode, sometimes called the overcharge mode. By this
time, the battery has achieved 85% to
95% of its full charge. As the battery continues to chargewith the voltage held
constant at 14.6 Vthe charging current
begins to drop. When the charging current falls to 30 mA, the three-mode
charger switches to float mode and lowers the applied voltage to 13.8 V . At
13.8 V, the battery becomes self-limiting,
drawing only enough current to offset its
normal self-discharge rate. This works
greatuntil you attach a light load to the
battery, such as turning on the K2 receiver. The K2 receiver normally draws
about 220mA. When the charger detects
a load current above 30 mA, it’s fooled
into thinking that the battery needs charging, so it reverts to the absorption mode,
applying 14.6V to the ba ttery. If left in
this condition, the battery is overcharged,
shortening its service life.
UC3906-IC Chargers
Chargers using the UC3906 SLA
charge-controller IC work just like the
three-mode charger described earlier except that their return from float mode to
absorption mode is based on voltage
rather than current. Typically, once the
charger is in float mode it won’t return
to absorption mode until the battery voltage drops to 10% of the float-mode voltage (or about 12.4V). Although this is
an improvement over the three-mode
charger, it still has the potential for overcharging a battery to which a light load
is attached.
First, let’s look at the situation where
a UC3906-controlled charger is in absorption mode and you turn on the K2
receiver, applying a load. The battery is
fully charged, but because the load is
drawing 220 mA, the charging current
never drops to 30 mA and the charger
remains in absorption mode, thinking that
it is the battery that is asking for the current. As with the three-mode charger, the
battery is subject to being overcharged.
If we remove the load by turning off
the K2, the current demand drops below
30 mA and the charger switches to float
mode (13.8 V). When the K2 is turned
on again, because the charger is able to
supply the 220 mA for the receiver, the
battery voltage doesn’t drop, so the
charger stays in float mode and all is well.
However, if the transmitter is keyed (increasing the current demand), the charger
can’t supply the required current, so it’s
taken from the battery and the battery
voltage begins to drop. If we unkey the
transmitter before the battery voltage
reaches 12.4 V, the charger stays in float
From May 2001 QST © ARRL
Figure 1—Schematic of the SLA
charger. Unless otherwise specified,
resistors are 1/4-W, 5%-tolerance
carbon-composition or metal-film
units. Part numbers in parentheses
are Digi-Key (Digi-Key Corp, 701
Brooks Ave S, Thief River Falls, MN
56701-0677; tel 800-344-4539, 218681-6674, fax 218-681-3380;
www.digikey.com). Equivalent
parts can be substituted; n.c.
indicates no connection. (The
component designations for D1, D3
and J1 differ from QST style.)
C1, C2—2200 µF, 35 V electrolytic
(P5751)
C3, C6, C7, C80.1 µF, 50 V
metallized-film (104) (P4525)
C4, C522 µF, 25 V tantalum (P2051)
D1400 V, 4 A bridge rectifier
(KBL04)
D21N5245 Zener diode, 15 V,
500 mW (1N5245BDICT)
D3Bicolor LED, red/green
(160-1036)
D41N5820 Schottky diode
(1N5820DICT)
F10.25 A slow-blow fuse
(283-2267)
J12-pin header, PC mount
(S1011-02)
J22-pin connector, PC mount
(WM5224)
J33-pin connector, PC mount
(WM5225)
Q12N4401 NPN transistor (2N4401)
R11 kΩ, 1 W, 5% (1.0 KW-1)
R2240 Ω (240QBK)
R32.4 Ω, 1 W, 5% (2.4W-1)
R42.2 kΩ (2.2KQBK)
R512 kΩ (12KQBK)
R6150 kΩ (150KQBK)
R71 MΩ (1.0MQBK)
R83.6 kΩ (3.6KQBK)
R9, R1010 kΩ (10KQBK)
R11100 kΩ, 1/4 W, 1% (100KXBK)
R1216.2 kΩ, 1/4 W, 1% (16.2KXBK)
R1320 kΩ multiturn pot
(3296Y-203)
R14, R15680 Ω (680QBK)
T115 V ac, 666 mA (TE70043)
U1LM317T voltage regulator,
TO-220 case (LM317T)
U2LM555 timer (LM555CN)
U3LM78L12 voltage regulator,
TO-92 case (NJM78L12A)
U4LM358 dual op amp (LM358N)
U5LM336, 2.5 V voltage reference,
TO-92 case (LM336Z-2.5NS)
Misc: PC board (see Note 3); TO-220
heat sink (294-1036); five 1/4-inch,
#4-40 stand-offs (1892K); two fuseholder clips, PC mount (283-2335);
two-pin shunt (S9002); two-pin
connector housing (WM2111); threepin connector housing (WM2112);
four housing pins (WM2305);
enclosure
From May 2001 QST © ARRL
mode. Now it takes much longer for the
charger to supply the battery with the
power used during transmit than it would
have if the charger had switched to absorption mode.
SLA batteries must
be recharged on a
regular basis.
Let’s key the transmitter again, but this
time keep it keyed until the battery voltage drops below 12.4 V. At this point, the
charger switches to the absorption mode.
When we unkey the transmitter, we’re
back to the situation where the charger is
locked in absorption mode until we turn
off the receiver.
Why Worry?
So, why this concern about overcharging an SLA battery? At 13.8 V, the
battery self-limits, drawing only enough
current to offset its self-discharge rate
(typically about 0.001 times the battery
capacity, or 2.9 mA for a 2.9 Ah battery).
An SLA battery can be left in this floatcharge condition indefinitely without
overcharging it. At 14.6 V, the battery
takes more current than it needs to offset
the self-discharge. Under this condition,
oxygen and hydrogen are generated faster
than they can be recombined, so pressure
inside the battery increases. Plastic-cased
SLA batteries such as the PS-1229A have
a one-way vent that opens at a couple of
pounds per square inch pressure (PSI) and
release the gases into the atmosphere.
This results in drying the gelled electrolyte and shortening the battery’s service
life. Both undercharging and overcharging need to be avoided if we want to get
maximum service life from the battery.
Continuing to apply 14.6 V to a 12-V
SLA battery represents a relatively minor amount of overcharge and results in
a gradual deterioration of the battery.
Applying a potential of 16V or e xcessive bulk-charging current to a small SLA
battery from an uncontrolled solar panel
can result in serious overcharging. Under these conditions, the overcharging
can cause the battery to overheat, which
causes it to draw more current and result
in thermal runaway, a condition that can
warp electrodes and render a battery useless in a few hours. To prevent thermal
runaway, the maximum current and the
maximum voltage need to be limited to
the battery manufacturer’s specifications.
Design Decision
To avoid the potential of overcharging a battery with an automatic charger
locked up by the load, I decided to design my own charger, one that senses battery voltage rather than current in order
to select the proper charging rate. A
500-mA current limiter sets the maximum
bulk rate charge to protect the battery and
the charger’s internal power supply. Like
the three-mode chargers, when a battery
with a low terminal voltage is first connected to the charger, a constant current
of 500 mA flows to the battery. As the
battery charges, its voltage begins to
climb. When the battery voltage reaches
14.5 V, the charger switches off. With no
charge current flowing to the battery, its
voltage now begins to drop. When the
current has been off for four seconds, the
charger reads the battery voltage. If the
potential is 13.8V or less, the charger
switches back on. If the voltage is still
above 13.8 V, the charger waits until it
drops to 13.8 V before turning on. The
result is a series of 500-mA current pulses
varying in width and duty cycle to provide an average current just high enough
to maintain the battery in a fully charged
condition. Because the repetition rate is
very low (a maximum of one current
pulse every four seconds) no RFI is generated that could be picked up by the K2
receiver. Because the K2’s critical circuits are all well regulated, slowly cycling the battery voltage between 13.8 V
and 14.5 V has no ill effects on the transmitted or received signals.
Thermal runaway can
warp electrodes and
render a battery useless
in a few hours.
As the battery continues to charge, the
pulses get narrower and the time between
pulses increases (a lower duty cycle). Now
when the K2 receiver is turned on and
begins drawing 220 mA from the battery,
the battery voltage drops more quickly so
the pulses widen (the duty cycle increases)
to supply a higher average current to the
battery and make up for that taken by the
receiver. When the K2 transmitter is
keyed, it draws about 2 to 3 A from the
battery. Because the charger is current limited to 500 mA, it is not able to keep up
with the transmitter demands. The battery
voltage drops and the charger supplies a
constant 500 mA. The battery voltage continues to drop as it supplies the required
transmit current. When the transmitter is
unkeyed, the battery voltage again begins
to rise as the charger replenishes the energy used during transmit. After a short
time, (depending on how long the transmitter was keyed) the battery voltage
reaches 14.5 V and the pulsing begins
again. The charger is now fully automatic,
maintaining the battery in a charged
condition and adjusting to varying load
conditions.
The great thing about this charging
system is that during transmit the majority of the required 2 to 3 A is taken from
the battery. When you switch back to receive, the charger is able to supply the
220 mA needed to run the receiver and
deliver up to 280 mA to the battery to
replenish what was used during transmit.
This means that the power source need
only supply the average energy used over
time, rather than being required to supply the peak energy needed by the transmitter. (You don’t need to carry a heavy
3-A regulated power supply with your
K2.) As long as you don’t transmit more
than about 9% of the time, this system
should be able to power a K2 indefinitely.
Have you ever noticed that sometimes
when your H-T has a low battery and you
drop it into its charger you hear hum on
the received signals? This charger’s
power supply is well filtered to ensure
that there is no ripple or ac hum to get
into the K2 under low battery voltage
conditions.
Circuit Description
The charger schematic is shown in
Figure 1. I’ve dubbed the charger the
PCR12-500A, short for Pulsed-Charge
Regulator for 12-V SLA batteries with
maximum bulk charge rates of 500mA.
U1, an LM317 three-terminal voltage
regulator, is used as a current limiter,
voltage regulator and charge-control
switch. A 15-V Zener diode (D2) sets U1
to deliver a no-load output of 16.2 V. R3
sets U1 to limit the charging current to
500mA. When Q1 is turned on by the
LM555 timer (U2), the ADJ pin of U1 is
pulled to ground, lowering its output voltage to 1.2 V. D4 effectively disconnects
the battery by preventing battery current
from flowing back into U1. A Schottky
diode is used at D4 because of its low
voltage drop (0.4 V).
An LM358 (U4A) operates as a voltage comparator. U5, an LM336, provides
a 2.5-V reference to the positive input
(pin 3) of U4. R11, R12 and R13 function as a voltage divider to supply a portion of the battery voltage to pin 2 of
U4A. R13 is adjusted so that when the
battery terminal voltage reaches 14.5 V,
the negative input of U4A rises slightly
above the 2.5-V reference and its output
switches from +12 V to 0 V. When this
happens, the 1-MΩ resistor (R7) causes
the reference voltage to drop a little and
provide some hysteresis. The battery voltage must now drop to approximately
From May 2001 QST © ARRL
board is 1/4-inch higher than the one identified in the parts list and results in
slightly cooler operation of U1. The remaining parts are available from
Digi-Key.
Be sure to space R1 and R3 away from
the board by 1/ 4 inch or so to provide
proper cooling. R13 can be a single-turn
or a multiturn pot. You’ll probably find a
multiturn pot makes it easier to set the
cutoff voltage to exactly 14.5 V.
R13 Adjustment
The populated PC board fits comfortably inside
the LMB Perf-137 box, ready for final assembly
of the charger. You can see how easy it is to
assemble or disassemble the charger.
13.8 V before U4A turns back on.
U4B is a voltage follower. It pulls the
trigger input (pin 2) of U2 to 0V , causing its output to go to 12 V. U4B’s output
remains at 12 V until C5 has charged
through R6 (approximately four seconds)
and the trigger has been released by U4A
sensing the battery dropping to 13.8 V or
less. While the output of U2 is at 12 V,
emitter/base current for Q1 flows via R5
and Q1’s collector pulls U1’s ADJ pin to
ground, turning off the charging current.
The output of U2 also provides either
+12 V or 0 V to the bicolor LED, D3. R14
and R15 form a voltage divider to provide
a reference voltage to D3 such that D3
glows red when U2’s output is +12 V and
green when U2’s output is at 0 V. When
ac power is applied but U1 is switched off
and not supplying current to the battery,
D3 glows red. When U1 is on and supplying current to the battery, D3 is green. As
the battery reaches full charge, D3 blinks
green at about a four-second rate. As the
battery charge increases, the on time of
the green LED decreases and the off time
increases. A fully charged battery may
show green pulses as short as a half-second and the time between pulses may be
60 seconds or more.
T1, D1, C1 and C2 form a standard
full-wave-bridge power supply providing
an unregulated 20 V dc at 500 mA. U3,
an LM78L12 three-terminal regulator,
provides a regulated 12-V source for the
control circuits.
Note that the mounting tab on U1 is
not at ground potential. U1 should be
mounted to a heat sink with suitable elecFrom May 2001 QST © ARRL
trically insulated but thermally conductive
mounting hardware to avoid short circuits.
Suitable mounting hardware is included
with the PC board (see Note 4).
Other Bulk-Charge Rates
The maximum bulk-charge rate is set
by the value of R3 in the series regulator
circuit. The formula used to determine the
value of this resistor is R ohms = 1200 / ImA.
T1 must be capable of supplying the bulk
charge current and U1 must be rated to
handle this current. The LM317T used
here is rated for a maximum current of
1.5 A provided it has a heat sink sufficiently large enough to dissipate the generated heat. If you increase the bulk-charge
rate, you’ll definitely need to increase the
size of the on-board heat sink. Mounting
U1 directly to the housing (be sure to use
an insulator) may be a good option.
To check for proper operation and to
set the trip point to 14.5 V dc, we need a
test-voltage source variable from 12 to
15 V dc. A convenient means of obtaining this test voltage is to connect two
9-V transistor-radio batteries in series to
supply 18 V as shown in Figure 2. Connect a 1-kΩ resistor (R2) in series with a
1-kΩ potentiometer (R1) and connect this
series load across the series batteries with
the fixed-value resistor to the negative
lead. The voltage at the pot arm should
now be adjustable from 9 to 18 V. During
the following procedure, be sure to adjust the voltage with the test supply connected to the charger at J2 because the
charger loads the test-voltage supply and
causes the voltage to drop a little when
it’s connected.
Remove the jumper at J1 and apply ac
line voltage to the unit at J3. Turn R13
fully counterclockwise. D3 should glow
green. Connect the test voltage to J2 and
adjust R1 of Figure 2 for an output of
14.5 V. Slowly adjust R13 clockwise until D3 glows red. To test the circuit, wait
at least four seconds, then gradually reduce the test voltage until D3 turns green.
At that point, the test voltage should be
approximately 13.8 V. Slowly increase
the test voltage again until D3 turns red.
The test voltage should now read 14.5V .
If it is not exactly 14.5V , make a minor
adjustment to R13 and try again. The aim
of this adjustment is to have D3 glow red
Transformer Substitution
I selected T1 because of its small size
and PC-board mounting. You can substitute any transformer rated at 15 or 16 V
ac (RMS) at 500 mA or more. You may
find common frame transformers to be
more readily available. You can mount
such a transformer to an enclosure wall
and route the transformer leads to the
appropriate PC-board holes.
Construction
There is nothing critical about building this charger. You can assemble it on a
prototyping board, but a PC board and
heat sink are available. 4 The specially
ordered heat sink supplied with the PC
Figure 2—Test voltage source for the
battery charger. (The component
designation for the push-button switch
differs from QST style.)
just as the test voltage reaches 14.5 V.
To test the timer functioning, remove
the test voltage from J2 and set it for
about 15V . Momentarily apply the test
voltage to J2. D3 should turn red for approximately four seconds, then turn
green. The regulator is now calibrated and
ready for operation. Remove the test voltage and ac power and install the jumper
at J1.
A Suitable Enclosure
I used an 8×3×2.75-inch LMB Perf137 box (Digi-Key L171-ND) to house
the charger. An alternative enclosure is
the Bud CU482A Convertabox, which
measures 8×4×2 inches (available from
Mouser). If you use the Convertabox, be
sure to add some ventilation holes directly above the board-mounted heat
sink. The LMB Perf box comes with a
ventilated cover. If you are inclined to
do some metal work, you could build
your own enclosure using aluminum
angle stock and sheet and probably reduce the size to perhaps 8×3×2inches.
If you use a PC-board-mounted power
transformer, watch out for potential
shorts between the transformer pins (es-
pecially the 120-V ac-line pins) and the
case. If you use a metal enclosure, connect the safety ground (green) wire of the
ac-line cord directly to the case.
Operation
It is very important that this charger
be connected directly to the SLA battery
with no diodes, resistors or other electronics in between the two. The charger
works by reading the battery voltage, so
any voltage drop across an external series
component results in an incorrect reading and improper charging. For example,
the Elecraft K2 has internal diodes in the
power-input circuit, so it’s necessary to
add a charging jack to the transceiver that
provides a direct connection to the battery. Now I can leave my K2 connected
to the charger at all times and be assured
that its internal battery is fully charged
and ready to go at a moment’s notice.
Notes
1
Larry Wolfgang, WR1B, “Elecraft K2 HF
Transceiver Kit,” Product Review, QST, Mar
2000, pp 69-74.
2
Although this charger was designed specifically for use with the Power-Sonic PS-1229A
SLA battery used in the Elecraft K2 trans-
ceiver, its design concepts have wide ranging applications for battery operated QRP
rigs of all types.
3
Although it’s labeled a 12-V battery, the terminal voltage is nominally 13.8 V with no
load.
4
A PC Board (double sided, plated through
holes, solder masked and silk screened) and
heat sink are available from Intelligent Software Solutions, PO Box 522, Garrisonville,
VA 22463-0522. Price: $18 plus $1.50 shipping in the US and Canada.
Bob Lewis, AA4PB, became interested in
Amateur Radio during junior high school in
the late ’50s. With the encouragement of his
cousin, Al Krugler, K8DDX, Bob obtained his
Technician license (K8KNI) and spent most
of his time on 6-meter AM in the Detroit,
Michigan area. His early interest in Amateur
Radio resulted in a career in electronics, first
as a radio mechanic in the air-transport industry, followed by ten years in the Navy as
an aviation electronics technician. While in
the Navy, Bob found 6-meter activity to be a
bit sparse in the middle of the Atlantic Ocean,
so he upgraded to General, then Advanced
and finally, Extra class. He enjoys QRP,
PSK31 and homebrewing. Bob is retired from
Civil Service, currently working part-time for
an electronics consulting firm. You can contact him at Box 522, Garrisonville, VA 22463;
rlewis@staffnet.com.
From May 2001 QST © ARRL